1. Field of the Invention
The presently claimed device is a passive adaptive airfoil structural insert. The claimed device consists of an insert design that allows wind turbine blades, aircraft wings, and other aerodynamic structures to respond passively to aerodynamic loads for mitigating the effects of gust conditions leading towards structural weight reductions and performance improvements while still maintaining stable flight. The main discriminator of the presently claimed design allows for a reduced structural mass fraction in the wind turbine blades, aircraft wings, and other aerodynamic structures.
2. Description of Related Art
Adaptive structures have been a focus of attention for the last decade in their ability to offer aircraft, automotive, and wind turbine industries significant leaps in performance, whether it be reliability, speed, efficiency, etc. Several approaches, both passive and active, have been demonstrated for use in adaptive structures, but none compare to the Active and Passive Adaptive approaches currently presented. Examples of adaptive approaches include: Piezoelectric Uni/Bi-morph composites; compliant structures; rigid body, lumped compliance in adaptive structures; asymmetric, unbalanced composite laminates for bend/twist coupling; and inflatable structures using variable pressure, or piezoelectric actuation.
There have been many attempts to design a wing that changes shape, or morphs on command or at some predetermined design characteristic. Almost all of these attempts involve some form of active control to change the shape of the wing. The key discriminators between the prior art and the currently claimed device is the focus on reduced weight while maintaining aerodynamic characteristics and designs that allow non-linear deflections through passive morphing of the aircraft while in flight.
Prior Morphing Wings and Aerodynamic Shapes
The design of conventional fixed wing aircraft has historically balanced the conflicting requirements of multiple mission segments for a given application. The result is typically a vehicle that has average performance over a range of mission segments, such as takeoff, cruise, dash, loiter, and landing; or optimal performance in the mission segment within which the most time is spent; and below average performance in other mission segments. Conventional mechanisms that result in the geometric changes that enable operation over multiple mission segments, such as ailerons, elevators, rudders, flaps and slats, are normally part of control systems that ensure the vehicle will be able to operate within a desired flight envelope, but the implementation of these systems does not typically result in a design that can continually respond and adapt to changing environmental or aerodynamic conditions in the most efficient manner.
The development of new smart materials, together with the always present need for better performance, is increasingly prompting designers towards the concept of morphing aircraft. These aircraft possess the ability to adapt and optimize their shapes to achieve dissimilar, multi-objective mission roles efficiently and effectively. Motivations for such unmanned aircraft are birds that morph between loiter and attack missions by changing their wing configuration accordingly. Birds also use camber and twist for flight control. The Wright Brothers used wing warping as a seamless flight control in their first flying machine. Morphing wings for flight control bring new challenges to the design of control laws for flight.
There are typically three primary reasons for morphing; (1) improve the aircrafts performance over a wide range of flight regimes; (2) offer multi-role capability in a single aircraft system; (3) improve aircraft survivability by offering the ability to adapt and respond to environmental or vehicle conditions. These different applications are all regarded as morphing; however, each is very different in terms of the magnitude of the shape changes required and time constants necessary for these changes. Fortunately large changes (global shape change) for multi-role capabilities are only required at low frequency, and very fast changes for flight control (local shape change) only need to be small amplitude relative to the global changes in shape. This means that there is likely never going to be a single solution for a morphing aircraft, and the technology employed will be vastly different depending on the application required. However, all applications require that morphing achieve the objective of improved performance and/or capability. Previously, this improvement was at the expense of increased weight and complexity, and the performance improvement must account for this.
Large-scale shape changes for configuration morphing (that is significant planform changes) include wing extension, wing folding, and wing sweep. Significant aerodynamic performance gains were previously only achievable through large overall changes in the aircraft geometry via wing sweep, area, and/or span. The application of morphing to flight control usually involves small geometric wing changes such as the use of ailerons, elevators, flaperons, and so forth, as well as wing warping techniques to enhance the control authority of the aircraft. Prior art in both of these categories, shows that these medium to large-scale changes are obtained with complex and sophisticated mechanical devices, significantly increasing the installation and maintenance costs as well as the structural weight of the airframe. It is clear therefore, that substantial gains in these areas could be made if alternative methods to enact these changes were found. Basic morphing motions for seamless flight control include wing twist, wing chamber change, and asymmetric wing extension. The use of winglets as control effectors may also yield substantial benefits.
There are many challenges in the design of morphing aircraft: the integrity of compliant structures needs to be ensured, the system should be designed so the required actuation force is realizable, the skin has to be designed to give a smooth aerodynamic surface yet support the aerodynamic loads, the design process should be extended to encompass multiple flight regimes, engines need to be designed for efficient low and high speed operation, and control systems will have to cope with highly coupled control effectors.
HALE Platforms
High-altitude, long-endurance (HALE) unmanned air vehicles (UAVs) offer the potential to provide a significant role in command and control, communications, computers, intelligence, surveillance, and reconnaissance architecture envisioned for the Department of Defense and offer capabilities within the commercial sector, enabling new initiatives in environmental surveys, agricultural studies, communication architectures, wind energy, and many other applications. With the advent of Compass Cope, an Air Force sponsored program initiated in 1971 to evaluate the feasibility of creating a HALE platform, requirements for these platforms have evolved from an endurance exceeding twenty-four (24) hours at an altitude of 55,000 feet to an endurance up to five (5) years at altitudes possibly exceeding 100,000 feet.
There is a need for reduced wing loading to increase the platform endurance. One way of doing this is mitigate the effects of gust conditions, therefore reducing the design load requirements, resulting in lighter structures and a reduced structural mass fraction, which results in a lower wing loading.
Prior active approaches do not offer the degree of shape control, or the degree of tailorability in structural design to control the aerodynamic loads specific to the HALE aircraft application. The weight of the active approaches also precludes their use in HALE platforms due to the increased weight and complexity of the actuation and control systems. Furthermore, the passive approaches described in prior art typically result in a linear deflection curve, whereas the currently claimed approach offers a highly nonlinear, stepwise response to aerodynamic loads in addition to a high degree of tailorability.
The requirements associated with HALE aircraft design and performance place a premium on several factors including; aerodynamic efficiency, structural efficiency, propulsive efficiency, and most importantly, reliability. Structural efficiency is a key element in achieving the extreme endurance initiatives, which is highlighted in the performance of the HELIOS UAV, developed for NASA, which demonstrated successful flight at an altitude of 96,000 feet with a maximum wing loading of one pound per square foot. Future applications capable of achieving flight duration of five years or greater will likely possess wing loadings of 0.7 pounds per square foot (lbs/ft^2) or less making them more susceptible to atmospheric turbulence or gust loads when ascending or descending through the troposphere.
Current design approaches incorporate gust load estimations in the structural sizing, resulting in higher structural weights and potential structural failure if unpredictable flight conditions are experienced. Introduction of innovative methods for offsetting the effects of gust loads to significantly reduce the structural mass fraction by reducing the maximum design loads and enabling the desired wing loadings is highly desired. As a result of these long-term goals, opportunities exist to identify and develop unique solutions for reducing the structural mass fraction for HALE platforms.
The overarching goal of current programs is to enable higher altitude or longer endurance flight times of HALE aircraft. There are several ways this can be accomplished (inflatable structures, tensegrity, advanced composites, etc.), but for the passive adaptive approach the objective is to use traditional composite design approaches coupled to a tailored aerodynamic response to limit the aerodynamic loads that the aircraft will experience. For the case of HALE aircraft, the primary loads driving the structural design space are the gust loads experienced during ascent through the troposphere. Once the aircraft reaches cruise altitude above the stratosphere in the troposphere, the thermal conditions minimize the vertical gusts that aircraft would otherwise experience within the troposphere. The effect of the gust load is to increase the ultimate design loads that the HALE structure must withstand, therefore increasing the sizing of the individual structural components and increasing structural weight. By controlling the shape of the aircraft wing in the event of a gust, the effect of the gust loads can be alleviated, therefore decreasing the ultimate design loads driving the structural sizing and resulting weights. Shape control can be accomplished in one of two ways, passively or actively, but for the case of HALE aircraft, the needed low wing loadings further preclude the use of active systems due to the complexity of the control systems required and the added weight of distributed actuators for shape control. The ultimate approach to controlling gust loads and shape change is a highly-tailorable, passive approach that avoids the challenges and weight associated with controls, electrical wiring, and actuators in an active design that is highly susceptible to breakdowns in the active actuators.
HALE aircraft are a relatively new breed of aircraft that until recently, have not received the level of attention and challenging design requirements as those associated with other aircraft. When comparing the wing loadings of commercial aircraft (W/S=18.7 lbs/ft^2), Boeing's CONDOR HALE aircraft (W/S=2.3 lbs/ft^2), the U2 spy plane (W/S=4.7 lbs^2), and AeroVironment's HELIOS HALE aircraft (W/S=0.9-1.1 lbs/ft^2), a wing loading target of 0.6 lbs/ft^2, are pushing the limits of materials and structure design; therefore, techniques for controlling loads are needed. In response to the VULTURE program and the need to further reduce structural mass fraction, the key teams involved are focused on new structural concepts rather than controlling loads. Examples of their approaches include inflatable structures, tensegrity (“sticks and string”), or advanced three-dimensional composites. Each of these approaches is valid in its own right, but each one introduces a complexity in structure design and fabrication that may not result in the desired weight savings; however, little information is available regarding the scope of the work and the details of the various approaches due to the competitive nature of these projects.
Wind Turbines
Wind turbines provide a cost-effective, environmentally-friendly method of producing electricity. Most wind turbines are now made from composite materials. Many are fiberglass rather than carbon fiber, since fiberglass is more cost-effective. Carbon fiber composites have high specific strength, which means that they can carry substantial loads for a given structural weight. There are two primary problems: 1) damage caused by wind gusts to the wind turbine and generator and 2) heavy structures. Wind turbines must disengage from the generator above certain wind thresholds or risk damaging the entire system, from blade, to generator, to power inverter. Passive adaptive structures decrease the load that the system experiences at high wind speeds, thus enabling the system to stay engaged without risking damage to the system. The return on investment is two-fold uninterrupted power generation, reduced maintenance/repair expenses.
For the structural weight problem, structures are sized to resist worse-case wind gusts, which occur infrequently. If the worst-case wind gusts result in lower worse-case structural loads due to integrated passive adaptive technology, the structures can be sized smaller, and will weigh less. Therefore, the system will require less starting inertia, may possibly operate at lower wind speeds, and will probably have some efficiency gain across the operating range.
Active Morphing Wings
Active morphing wings means a wing that uses some form of internal mechanical force, such as a gear, motor, or similar device, to move the wing structure from one position to another. Reduction of structural mass fraction is the goal of passive adaptive morphing applied to HALE aircraft and for improved performance (i.e. speed, endurance, etc.). Active morphing, however, is typically focused on performance and not reduction of structural mass fraction.
U.S. Pat. No. 4,796,192, issued on Jan. 3, 1989 to Lewis, discloses a method to reduce the bending moment load on aircraft wing root structures. The disclosed method uses measurements of the aircraft flight parameters and control surface configuration to calculate a theoretical value of the bending moment load on the wing root. If this theoretical value is above a predetermined threshold, a flight computer will command motors to cause the upwards deflection of the outboard ailerons to reduce the actual bending moment load on the wing root. The use of a flight computer, sensors, and automated actuation of ailerons adds mass and complexity to the aircraft, while the claimed reduction of mass and complexity from the wing root structure provides for an ultimate reduction in aircraft gross weight. However, the control system and materials used are still significantly heavier than the currently presented devices and does not allow for a significant shape change in the wing design during flight.
The general concept of rotor blades that passively adapt to the incident wind loading is not new. Mechanisms that adjust blade angle of attack in response to the thrust loading were quite popular in the early days of the modern wind energy push of the late twentieth century. Approaches to and objectives of various systems are quite varied. One effort regulated power with a centrifugally loaded mass on an elastic arm. Twisting to feather in response to increasing winds is a known potential means to reduce the dynamic loading on the blades, and hence the rest of the system. Load reductions have been demonstrated by linking a pitch control system to flapwise blade loads using simple integral control and standard rotor blade theory. Yet all of these methods are not as successful in alleviating gust loads, either due to the complexity of the design or the failure to establish a significant reduction in the mass fraction of the wing.
Morphing Skins
Morphing skins must transition between rigid substructures, where they are connected, and the high-strain morphing zone. This requires the skin to transition between zero strain at the substructure to the high strain (10% or greater) in the morphing zone. Currently this transition takes place over a short length, and there is a large discontinuity in stiffness where the attachment zone meets the high-strain morphing zone. This jump in elastic modulus creates a high-stress concentration along the interface and leads to localized failure after a few cycles of activation.
Passive Morphing Wings
Some attempts at designing a morphing wing use passive geometry adaptation in an attempt to alter the wing design based on loading and/or deformation of the wing tips. U.S. application Ser. No. 10/551,406, publication number US 2007/0036653, published on Feb. 15, 2007 and filed by Bak et al. describes a design concept by which the power, loads and/or stability of a wind turbine may be controlled by typically fast variation of the geometry of the blades using either active controls, such as actuators, or passive controls which arise from the changes in the load or deformation of the blades. Changing the shape of the wing by a combination of the two methods is possible. The passive controls comprise oblique changes in the geometry obtained from influence of blade deformation, e.g. a change in effective camber from blade flap-wise bending or from pressure fluctuations from the interaction with the flow. Bak has a bend/twist coupling, which means that the deflections are going to be limited via the stiffness of the structures while maintaining the same load path. In the currently disclosed passive design, there is a changing of the camber of the wing, but not twisting the wing. There is a higher degree of tailorability (i.e. mitigate multiple gust conditions due to the stepwise, non-linear response that can be tailored both spanwise and chordwise). The bend/twist coupling of the Bak approach is a linear deflection response.
U.S. Pat. No. 6,161,801, issued on Dec. 19, 2000, to Kelm et al., discloses a method to reduce the bending moment on wings. In this method, wing control surfaces are placed in a certain configuration during takeoff and landing phases to reduce the lift generated by the outboard portions of the wing, consequently reducing the bending moment on the wing. This method expressly neglects sensing the aircraft flight parameters, and instead assumes that the aircraft will encounter flight conditions generating aerodynamic loads in excess of the wing bending moment threshold. This event is precluded by the takeoff and landing configurations. This method does not require an additional flight computer, sensors, or automated flight surface control, and this method does not require rapid sensing of the occurrence of a wind gust and rapid actuation of control surfaces to try to instantaneously counteract a wind gust as it occurs. This is purely flight controls and not really an “adaptive” wing. They are simply setting control surfaces in a position to offset loads towards the tip of the wing and increase the lift at the root to reduce the maximum root bending moment.
U.S. Statutory Invention Registration No. H2057 H, published Jan. 7, 2003, issued to Veers et al., discloses an entirely passive method of reducing the aerodynamic load on a wind turbine blade. The disclosed blade is made of a composite in which the fiber orientation causes the blade to twist around its spanwise axis if it is bent about its chordwise axis, a behavior known as “bend-twist coupling.” As the lift force generated by the blade increases, that force will cause the blade to bend outwards around its chordwise axis. The blade will then twist, reducing the angle of incidence and consequently the lift force generated by the blade. This passive system is used to control the loads experienced by the blade.
United Kingdom Patent Application No. GB 2,129,748, published on May 23, 1984, issued to Klug, discloses a passive system to mitigate gust loads on a wing. Inside the wing structure is an open channel between the upper and lower surfaces. The channel opening on the lower surface is covered by a hinged flap. During a gust event, the additional free-stream velocity increases the lift generated by the wing. If this additional lift force exceeds the wing structure load threshold, the flap on the lower wing surface will open, allowing the flow to pass through the channel and into the low-pressure region on the upper wing surface. The sudden introduction of high-pressure flow normal to the boundary layer induces flow separation, consequently reducing the lift force generated by the wing.
U.S. Pat. No. 7,384,016 to Kota et al., issued on Jun. 10, 2008, discloses that a variation in the contours of a first and second compliant surface is produced by a compliant frame having a first resiliently variable frame element with a corresponding first outer surface and a first inner surface, and a second resiliently variable frame element having a corresponding second outer surface and a second inner surface. The first and second outer surfaces communicate with their respective counterparts of the first and second compliant surfaces. A linkage element having a predetermined resilience characteristic is coupled at a first end thereof to the first inner surface and at a second end thereof to the second inner surface. A frame coupler joins the first resiliently variable frame element to a support element. An actuator applies a force to the second resiliently variable frame element with respect to the support element, resulting in a corresponding variation in the contour of the first and second compliant surfaces. This active approach still requires significant actuation means and does not address the reduced mass fraction or improved performances that the currently presented devices do.
The above disclosures teach several methods to accomplish substantially the same goal: the reduction of aerodynamically induced bending moment in the wing structure. Between the two passive methods, Veers & Klug, there is a great difference in how that goal is accomplished. The “bend-twist coupling” method disclosed by Veers effects a continuous, linear geometric shape change. The greater the load, the more twist the blade will exhibit, consequently decreasing the load and decreasing the degree of twist. The decreased twist will allow greater load, which will again increase the degree of twist. The continuous nature of the relationship between load and twist allows for oscillation between over and under-compensating the load.
Klug discloses a discontinuous, stepwise response to aerodynamic load. A certain pressure value will pull the flap open, allowing the high-pressure flow from under the wing to separate the boundary layer on the upper surface. This will suddenly reduce the lift generated by the wing. After the gust event, the flap will close, allowing the flow to reattach and quickly recover the lift generated by the wing. Since the flap opens at only one pressure value, and the pressure required to close the flap is much higher, there is no risk of aeroelastic oscillation. This method is not without shortcomings. Separating the flow between channel openings upstream of the outboard ailerons and control surfaces may render these control surfaces ineffective. The system is also at greater risk for failure as the channels can clog with airborne debris or ice accretion from captured condensation or rainwater.
Dynamic Modulus Resins
Dynamic Elastic Modulus Resins (DMR) are resins whose elastic modulus changes with a change in temperature of the resin. One such DMR is Shape Memory Polymer (SMP). Shape memory materials were first developed about twenty-five (25) years ago and have been the subject of commercial development in the last fifteen (15) years. Shape memory materials derive their name from their inherent ability to return to their original “memorized” shape after undergoing a shape deformation. There are principally two types of shape memory materials, shape memory alloys (SMAs) and shape memory polymers (SMPs).
SMAs and SMPs that have been pre-formed can be more easily deformed to a desired shape above their glass transition temperature (Tg). The SMA and SMP must remain below, or be quenched to below, the Tg while being restrained in the desired shape to “lock” in the deformation. Once the deformation is locked in, the SMA, because of its crystalline network, and the SMP, because of its polymer network, cannot return to a relaxed state due to thermal barriers. The SMA and SMP will hold their deformed shapes indefinitely until they are heated above their Tgs, whereupon the SMA and SMP stored mechanical strain is released and the SMA and SMP return to their pre-formed, or memory, states.
There are principally two types of plastics, thermoset resins and thermoplastic resins, each with its own set of unique characteristics. Thermoset resins, for example, polyesters, are liquids that react with a catalyst to form a solid, and cannot be returned to their liquid states, and therefore, cannot be reshaped without destroying the polymer networks. Thermoplastics resins, for example PVC, are also liquids that become solids. But unlike thermoset resins, thermoplastics are softened by application of heat or other catalysts. Thermoplastics can be heated, reshaped, heated, and reshaped repeatedly.
SMPs used in the presently disclosed method and devices are unique thermosetting polymers that, unlike traditional thermosetting polymers, can be reshaped and formed to a great extent because of their shape memory nature and will not return to a liquid upon application of heat. Thus, by creating a shape memory polymer that is also a thermosetting polymer, designers can utilize the beneficial properties of both thermosetting and thermoplastic resins while eliminating or reducing the unwanted properties. Such polymers are described in U.S. Pat. No. 6,759,481 issued to Tong, on Jul. 6, 2004, which is incorporated herein by reference. Other thermoset resins are seen in PCT Application No. PCT/US2005/015685 filed by Tong et al. on May 5, 2005, and PCT Application No. PCT/US2006/062179, filed by Tong, et al. on Dec. 15, 2006, of which both applications are incorporated herein by reference.
Unlike SMAs, SMPs exhibit a radical change from a normal rigid polymer to a flexible elastic and back on command. SMA would be more difficult to use for most applications because SMAs do not have the ease in changing the activation temperature as do SMPs, and SMAs are limited to low maximum strain values. SMAs would also have issues with galvanic reactions with other metals, which would lead to long-term instability. The current supply chain for SMAs is currently not consistent as well. SMP materials offer the stability and availability of a plastic and are more inert than SMAs. Additionally, when made into a composite, SMPs can offer similar mechanical properties to that of traditional composites used in the aerospace industry. SMPs and SMP composites can also achieve high strain levels. Throughout this disclosure SMP and SMP composites are used interchangeably as each can be replaced by the other, depending on the specific design requirements to be met.
Composites
The term “composite” is commonly used in industry to identify components produced by impregnating a fibrous material with a thermoplastic or thermosetting resin to form laminates or layers. Generally, polymer composites have the advantages of weight-saving, high-specific mechanical properties and good corrosion resistance, which make them indispensable materials in all areas of manufacturing. Because SMPs are resins, they can be used to make composites, which are referred to in this application as SMP composites.
Advanced composites, containing continuous fibers dispersed in a resin matrix material, are widely used in aerospace, sports equipment, infrastructure, automotive, and other industries, both as primary and secondary load-bearing structures. These composite materials derive their excellent mechanical strength, stiffness, and other properties from a combination of the resin and reinforcement fibers used. The addition of reinforcements such as continuous fiber, fiber mats, chopped fibers, fiberglass, nanoparticles and other similar material is known. Even with nanoparticles, like carbon nanotubes and carbon nano-fillers, a small amount of these nano-fillers could dramatically alter the properties of a matrix resin.
Composites have excellent mechanical properties. Like any material, they will yield or fail when limits are exceeded. These failures are prevented by physically limiting the load and displacement that the buckling members experience and by choosing a resin that is tough enough to survive with these particular limits.
This problem is of great concern because of the widespread and intensive use in modern society of polymers and polymer composites in product components. Traditional approaches to increasing the reliability of polymeric based components and products have included a focus on suitable design enhancements and the use of incrementally improved plastics.
Buckling Members and Structures
Structural members will exhibit buckling when compressed. Characterized as a failure mode, buckling is the tendency of compressed beams to suddenly exhibit large scale deformations under a load equal, or less than that, which initiated the failure. Beams loaded in tension do not exhibit buckling and can support greater loads than the same beam loaded in compression. For this reason, load-bearing structures have historically been designed to avoid buckling by bearing loads in compression. Aircraft structures are specifically designed in many cases to carry compressive loads without buckling. In most cases this is more important than tensile loads.
A post-buckled beam, which is a beam that has been sufficiently compressed to induce buckling, will exhibit greater deformation than the same beam in a non-buckled state under equal load. A post-buckled beam exhibits greater strain per stress than a non-buckled beam. This behavior does not facilitate the support of loads by a structure.
One of the few designs that actually utilizes buckling in a structure is described in U.S. Pat. No. 5,409,200 issued on Apr. 25, 1995 to Zingher et al. Zingher describes a thin sheet of metal which is pattern-folded and joined to produce a ray of compression springs. Each spring will exhibit constant force characteristics over a useful range of deflections to allow the array to apply nearly constant specified forces to closely spaced items which may be of varying size or height. As the springs are loaded from a relaxed state the rate of force increase per unit of increased deflection is initially high, tapering off to nearly zero (0) force increase with subsequent increases in deflection. This region of minimal force increase per unit of increased deflection (i.e. a near constant force band) extends over a useful range of deflections. The springs are self guiding and balanced, producing no lateral force on a perpendicularly applied load.
A second embodiment utilizing buckling is disclosed in U.S. patent application Ser. No. 11/407,535, publication number US 2007/025821 filed by Son et al. on Apr. 20, 2006. Son describes that the elements of the structure can be made to buckle or collapse so as to give the user the sensation of a “click” or “snap” upon the compression of the sensor. That function is valuable to allow the user to have a tactile confirmation of the completion of the process of button activation. It is achieved by providing the geometric elements of the compressible dielectric structure with limited column strength or a built-in buckling means as described later. This limits their column strength and encourages their collapse upon reaching a threshold pressure. Upon compression of the top surface by the user, the strips initially compress without collapse under light loads, but then collapse under higher load and the surface of the device buckles to one side after exceeding a predetermined threshold value. That value is easily adjustable by varying the properties of the material and the dimensions of the strips. Upon collapsing, the structure provides for instant movements of one surface closer to the other that can be used to provide a perception to the user that the button is fully compressed and no further compression is needed or useful.
While the design elements of Zingher are designed to buckle they are also designed to provide no support to any structural members causing them to buckle. This lack of support is the principal reason why engineers and architects do not design structural elements to buckle.
Critical to the performance of the springs in Zingher is that the spring of force must be produced by the entire length of the side member or leg and not concentrate to a small portion of the leg. To achieve this it is imperative that the profile of the legs be a gradual arc with no abrupt form changes or defects in the active region. Prior art springs could not simultaneously achieve a dense array and near constant force over a wide deflection. An array of the little washers can provide large density but only in a range of near constant force. A common tape measure contains a coiled metal tape which may be considered a spring with a constant tensile force over a very wide range of extension. This spring is incompatible with the dense array. Still other designs for buckling beam springs are not compatible with an array sheet structure and not compatible with fabrication like a printed circuit.
The exploitation of beam buckling behavior has enabled micro scale devices, tactile switches, and constant force springs. In none of these applications does the buckling beam, at any point, resist a load except with deflection. These applications exploit the fact that buckled beams exert a smaller load than the same unbuckled beam; the unbuckled beam's resistance to load is never required or utilized. All of the previously mentioned documents teach applications where the dimensions, deformations, and loads encountered are small in scale.
The exploitation of buckling beam behavior would incorporate the support of load by the member in the unbuckled state; the member would meaningfully contribute to structural integrity. The same member could then be buckled to provide constant spring back force, a sudden change in resistance to deformation, or allow deformation above a certain magnitude of load. This structure would be beneficial because of the incorporation of buckling and non-buckling phases. Also, the loads, deformations, and dimensions would be far greater than those encountered in the background art.
Thus, while the prior art has proposed systems for reducing wind gust loads, such systems have not come into practical use. That is because the majority of the commercially viable systems have all been intended to actively sense and counteract the effects of a wind gust as it occurs so as to reduce the actual instantaneous wind gust loading; operation in that manner places very high demands on such a system in actual practice. Namely, since the wind gusts causes very rapid and sudden variations in the loads applied to the wings, the sensors must react very rapidly and precisely, and the active measures such as actuation of control surfaces to counteract such wind gusts loads must be similarly rapid.
One primary advantage of a morphing platform would be the increased cost effectiveness of aircraft through eliminating the need for multiple, expensive, mission specific aircraft. However, from current trends in this research area, it is clearly evident that the practical realization of a morphing structure is a particularly demanding goal with substantial effort still required. This is primarily due to the need of any proposed morphing airframe to possess conflicting abilities to be both structurally compliant to allow configuration changes but also be sufficiently rigid to limit aeroelastic divergence. The currently disclosed devices present solutions to all of the problems mentioned.